1. Field of the Invention
The present invention relates to a voltage-controlled oscillator (VCO) and, more particularly, to a voltage-controlled oscillator using a current feedback network.
2. Discussion of Related Art
The voltage-controlled oscillator is a component used as a local oscillator in many wireless communication devices. As a conventional voltage-controlled oscillator, there are a Collpits oscillator that uses capacitor feedback, a Hartley oscillator that uses inductor feedback, and a Clapp oscillator that modifies the Collpits oscillator in terms of frequency stability.
Among these, the Hartley oscillator is in need of an inductor with a high Q-factor so that it is rarely used as a commercial product, while the Clapp oscillator and the Collpits oscillator are widely and commercially used due to good frequency stability and easy frequency tuning.
The Clapp oscillator and the Collpits oscillator generate negative resistance using the capacitor feedback. Therefore, due to a parasitic resistance component or a resistance component of the capacitor accompanied at the time of fabricating an oscillator circuit, the negative resistance may not be generated. Recently, in the commercial VCO products, a buffer is connected to an output of the Collpits oscillator or the Clapp oscillator to increase output power and improve a degree of isolation from the external load, thereby preventing degradation of the quality factor in the VCD circuit. However, when the buffer is connected to the output, the circuit becomes complicated so that a problem occurs that unpredictable parasitic components grow larger. Therefore, due to the unpredictable parasitic components, the oscillation may not be made at a designed frequency or the oscillation may not be made at all, which leads to a cumbersome task of tuning capacitors, resistors or inductors. It takes another time and money so that it is difficult to reduce the cost. Furthermore, since the tuning task cannot be performed in a semiconductor integrated circuit, yield is significantly reduced.